Polyvinyl alcohol is a well-known polymer and is generally commercially available in many forms for a variety of end uses.
Polyvinyl alcohol cannot readily be produced directly from vinyl alcohol. Instead, polyvinyl alcohol is produced on a commercial scale by polymerizing vinyl acetate (with optional comonomers) to generate polyvinyl acetate, after which the acetate groups are hydrolyzed to hydroxyl groups in varying degrees. Several different hydrolysis methods are well known and can be used for this purpose.
The present invention relates to polyvinyl alcohol generated by the slurry alcoholysis of vinyl acetate, which produces vinyl alcohol particulates of unique morphology due to the conditions maintained during the alcoholysis, as discussed below. Properties and applications of various polyvinyl alcohols (hydrolyzed polyvinyl acetates) produced by the slurry alcoholysis process are in a general sense known.
Polyvinyl acetate, the starting material for polyvinyl alcohol, is commercially produced by the free radical polymerization of the vinyl acetate monomer in the presence of a polymerization catalyst. The solvent commonly used in the commercial polymerization of vinyl acetate is methanol. The polymerization is conducted in the temperature range of 10° C. to 80° C. The lower end of the polymerization range is known to give products with improved properties. The percent conversion of vinyl acetate to polyvinyl acetate can vary over a wide range. Though conversions ranging from 20% to 100% have been found satisfactory, commercially at least about 30% conversion is preferable. The degree of polymerization of polyvinyl acetate varies anywhere from about 400 to about 10000.
The alcoholysis of the polyvinyl acetate to polyvinyl alcohol is typically performed in methanol in the presence of an alkali catalyst such as sodium hydroxide or sodium methylate, such as disclosed in U.S. Pat. No. 2,734,048. The major products of the alcoholysis reaction are polyvinyl alcohol and methyl acetate.
The alkali catalyzed alcoholysis of methanol solutions of polyvinyl acetate is typically conducted at temperatures between about 55° C. to about 70° C. In this temperature range, three distinct phases occur successively as the alcoholysis reaction proceeds. Starting as a homogeneous solution the solubility of the polyvinyl acetate in methanol decreases as increasing numbers of acetate groups are converted to hydroxyl groups. When the alcoholysis reaches about 40-50%, the polymer partially precipitates. The insoluble material takes the form of a gel of polymer molecules solvated with methanol. As the solubility decreases by further alcoholysis, the gel becomes tougher and begins to reject the associated solvent molecules. When the alcoholysis is completed, the polymer and solvent are mutually insoluble. If this gel is allowed to stand undisturbed, alcoholysis proceeds and the product is obtained in a massive, unworkable form. However, if the gel is worked mechanically (agitated) during this range above about 40% alcoholysis, the polymer will break down finely divided solid insoluble in the alcohol. The collapsing gel traps and sticks together with the fine particles from the previous alcoholysis cycle producing polyvinyl alcohol of a unique morphology. The unique morphology is a “popcorn” type particle made up of agglomerates of individual polyvinyl alcohol particles, such as shown in FIG. 1, as compared to particles made by other conventional processes, such as shown in FIG. 2.
Ultimately, the polyvinyl alcohol produced is not soluble in the methanol and methyl acetate solvent system and is obtained as a slurry.
Once the desired degree of alcoholysis (hydrolysis) is attained, the catalyst is neutralized with an acid such as acetic acid. The degree of hydrolysis refers to the percentage of acetate groups that are converted to hydroxyl groups. For most commercial polyvinyl alcohol, the degree of hydrolysis varies from 70% to about 100% (maximum).
Many of the important uses of polyvinyl alcohols are in the form of aqueous solutions; however, preparation of an aqueous solution, particularly at high concentration, can be challenging. Slurrying the polyvinyl alcohol in water at room temperatures and then heating the slurry to the temperature necessary to achieve the dissolution of the polymer is a common way of doing so. Polyvinyl alcohols with a degree of hydrolysis greater than 98% are relatively insoluble in water at temperatures below 70° C. and the characteristics of their room temperature water slurry can vary considerably depending on the conditions maintained during the alcoholysis of the parent polyvinyl acetate and the heat history applied to the polyvinyl alcohol. Polyvinyl alcohols which are 70-90% alcoholyzed are generally soluble in water at room temperature and are, consequently, difficult to slurry in water at room temperature.
In order to improve the cold-water slurrying properties of highly-hydrolyzed polyvinyl alcohols produced by an alcoholysis process such as disclosed in U.S. Pat. No. 2,734,048, U.S. Pat. No. 3,497,487 and U.S. Pat. No. 3,654,247 teach to heat treat such polyvinyl alcohols in specific liquid media.
As disclosed in U.S. Pat. No. 3,497,487, the media is a preferably an anhydrous 2-20% solution of acetic acid in a solvent comprising one or both of methanol and methyl acetate. A particulate polyvinyl alcohol is dispersed in the media and heated to a temperature of generally 50-150° C. (preferably from the atmospheric reflux temperature of the mixture up to 140° C.) for a time dependent upon the temperature (higher temperatures require shorter times).
As disclosed in U.S. Pat. No. 3,654,247, the media is methanol and optionally small amount of water and/or a 2-5 carbon atom alcohol, ester, ketone, ether, or certain other specified types of hydrocarbons. A polyvinyl alcohol slurry is mixed with other components to form the desired media, then heated to a temperature of generally 70-190° C. (preferably from 90-150° C.) for a time dependent upon the temperature (higher temperatures require shorter times).
In commercial continuous slurry alcoholysis processes to produce highly-hydrolyzed polyvinyl alcohol, this heat treatment is present and performed by pumping the neutralized polyvinyl alcohol slurry to a heat treatment vessel, where it is typically heated at temperatures of 110° C. or greater.
From the heat treatment vessel, the slurry is cooled and the polymer is separated from the methanol/methyl acetate solvent system to generate a polyvinyl alcohol cake. The polyvinyl alcohol cake is then typically washed with methanol to reduce ash content and other contaminants (purified), then the resulting polyvinyl alcohol particles are filtered and dried to a particulate product which is desirably a free-flowing powder.
This ultimately results in a highly crystalline polyvinyl alcohol product having certain particle size, surface area and bulk density, low warm-water solubility and low swelling volume properties.
There are, however, some desired end-use applications where the low warm-water solubility of these highly-crystalline polyvinyl alcohols are disadvantageous, for example, in certain adhesive, paper coating, emulsion stabilizer, pulp and various fiber end-uses.
It would, therefore, be desirable to provide a substantially fully-hydrolyzed polyvinyl alcohol that has low cold water solubility so that slurries can be prepared, but good warm water solubility to ease the formation of aqueous solutions, as well as a continuous process for producing such substantially fully-hydrolyzed polyvinyl alcohols that can be operated commercially.